2. Prions
Prions are infectious particles that contain no nucleic acids
Prions, so-called because they are proteinaceous, are infectious particles, smaller
than viruses, that contain no nucleic acids (neither DNA nor RNA).
Historically, the idea of an infectious agent that did not use nucleic acids was
considered impossible, but pioneering work by Nobel Prize-winning biologist Stanley
Prusiner has convinced the majority of biologists that such agents do indeed exist.
3. Prions
Fatal neurodegenerative diseases, such as kuru in humans and bovine spongiform
encephalopathy (BSE) in cattle (commonly known as “mad cow disease”), were
shown to be transmitted by prions.
The disease was spread by the consumption of meat, nervous tissue, or internal
organs between members of the same species.
4. kuru
Kuru, native to humans in Papua New Guinea, was spread from human to human
via ritualistic cannibalism.
BSE, originally detected in the United Kingdom, spread between cattle by the
practice of including cattle nervous tissue in feed for other cattle.
Individuals with kuru and BSE show symptoms of loss of motor control and
unusual behaviors, such as uncontrolled bursts of laughter with kuru, followed by
death.
Kuru was controlled by inducing the population to abandon its ritualistic
cannibalism.
5. Bovine spongiform encephalopathy (BSE)
On the other hand, BSE was initially thought to affect only cattle.
Cattle that died of BSE had developed lesions or “holes” in the brain, causing the
brain tissue to resemble a sponge.
Later on in the outbreak, however, it was shown that a similar encephalopathy in
humans known as variant Creutzfeldt-Jakob disease (CJD) could be acquired
from eating beef from animals with BSE, sparking bans by various countries on
the importation of British beef and causing considerable economic damage to the
British beef industry.
BSE still exists in various areas. Although a rare disease, individuals that acquire
CJD are difficult to treat.
The disease spreads from human to human by blood, so many countries have
banned blood donation from regions associated with BSE.
6. Pathogenesis
The cause of spongiform encephalopathies, such as kuru and BSE, is an infectious structural
variant of a normal cellular protein called PrP (prion protein).
It is this variant that constitutes the prion particle. PrP exists in two forms:
PrPc, the normal form of the protein
PrPsc, the infectious form
Once introduced into the body, the PrPsc contained within the prion binds to PrPc and converts it to
PrPsc.
This leads to an exponential increase of the PrPsc protein, which aggregates. PrPsc is folded
abnormally; the resulting conformation (shape) is directly responsible for the lesions seen in the
brains of infected cattle.
7.
8. Viriods
Viriods are a group of virus-like pathogens much smaller and distinctly
different from viruses, consists solely of small single stranded circular
RNA molecule and have no protein coat.
9. Structure
Viriods are covalently closed, circular, ssRNAs, about 250-370 nucleotides long.
The circular RNA normally exist as a rod-like shape due to intrastrand base pairing, which
forms double stranded regions with single-stranded loops.
The RNA of a viriod doesn’t encode any gene products;so they can’t replicate themselves.
It is believed to adopt the host RNA Polymerase for this purpose which synthesizes a
negative RNA strand.
Then it serves as a template for the synthesis of the other viriod RNAs
10. DISEASES Caused BY VIRIODS
Viriods causes over 20 different plant diseases of which the most studied are
1. Potato spindle-tuber disease
2. Exocortis disease of citrus trees
3. Chrysanthemum stunt disease
The only human disease caused by viriods is Hepatitis-D
11. Pathogenicity
Though the pathogenicity of viriods is not well understood
It is presumed that viriods cause disease by triggering a eukaryotic response called RNA
SILENCING, which normally functions to protect against infection caused by double stranded
viruses, by degrading it.
Viriods may usurp this response by hybridizing to specific host mRNA molecules to which they
have a complementary sequence forming Hybrid viriod:host mRNA ds molecule which will elicit
RNA silencing.
This results in destruction of the host message and thus, silencing of the host gene. This leads
to disease in the host plant.
12. Potato spindle tuber viriod
Potato Spindle Tuber Viroid (PSTV) is the first recognized viroid.
PSTV is infectious RNA of low molecular weight, approximately 100,000 Daltons.
The RNA is single stranded molecule of 359 nucleotides with extensive regions of base pairing.
Under the electron microscope, purified but denatured PSTV appears as short strands about 50
nm long and thickness of double stranded DNA PSTVd is a small, circular RNA molecule.
The natural hosts are potatoes (Solanum tuberosum) and tomatoes (Lycopersicon esculentum).
All potatoes and tomatoes are susceptible to PSTVd and there is no form of natural resistance.
13. Hepatitis D
Hepatitis D is a disease caused by a viriod called Hepatitis delta virus(HDV).
HDV is considered to be a subviral satellite because it can propagate only in the presence of
HBV.
The HDV genome exists as an enveloped negative sense, single stranded, closed circular
RNA.
Because of a nucleotide sequence that is 70% self-complementary, the HDV genome forms a
partially double stranded RNA structure that is described as rod-like.
With a genome of approximately 1700 nucleotides, HDV is the smallest viriod known to infect
animals.
